Voltage controlled oscillator (VCO) circuits are used in communication devices as the means of generating the desired frequency of operation. It is necessary to tune the VCO, usually as part of a phase locked loop (PLL) circuit, so that the communication device will lock on the desired frequency.
The bandwidth of a VCO is one of the most important parameters in VCO designs, however many types of compromises are often incurred due to the lack of bandwidth which generally relates back to lack of control voltage range. In many 3 volt radio designs, for example, the control voltage available to steer the VCO can be as little as 1 volt. This reduction in available control voltage is due to noise restrictions, tolerance restrictions, as well as temperature restrictions. Both noise and tolerance restrictions are fairly well understood in the art, and VCO designs are optimized with respect to these parameters. Temperature tolerance restrictions, on the other hand, are generally compensated for by narrowing the control voltage margins at both the low end and high end of the control voltage range which again limits the VCO's performance.
In many VCO designs wider bandwidth is considered desirable, however the disadvantage to increasing the bandwidth is that it makes the Ko(VCO gain parameter in MHz/V) much larger than it needs to be. The larger Ko tends to compromise the electrical performance of the VCO in terms of sideband noise and hum-and-noise. In the past, many VCOs were frequency-tuned which required some form of manual or factory level adjustment. One method used to eliminate frequency-tuning is to design the VCO frequency to a much wider bandwidth than necessary, but again, this has the disadvantage of making the Ko larger and degrading circuit performance. Voltage multiplier circuits can also be used to widen the VCO bandwidth by way of increasing the control voltage range and without necessarily increasing the Ko, but these circuits add unwanted complexity and cost to the circuit design.
Accordingly, there is a need for an improved VCO tuning apparatus and technique which provides for a wider control voltage range to be used which will enable improved performance in terms of noise, bandwidth, and other VCO parameters, without severely impacting the Ko.